1. Field of the Invention
This invention relates to machine lubrication systems, and more particularly, to means for providing lubrication to a differential bearing mounted between relatively rotating concentric shafts.
2. Description of the Prior Art
In the modern gas turbine engine, compressor and turbine rotors are attached to a central rotating shaft, and the turbine portion of the engine powers the compressor portion through this central shaft. Typically, this turbine shaft is positioned by bearing assemblies attached to stationary support housings. Due to the nature of the operation of the gas turbine engine, the turbine shaft necessarily rotates at high speeds. Consequently, the bearing assemblies must be lubricated and cooled with a liquid lubricant during engine operation. The lubricant is usually provided by nozzles that inject oil into the bearing assembly during engine operation.
This relatively simple and effective means of lubrication has been complicated by modern dual rotor turbine construction. Dual rotor turbines utilize two independent rotor systems, one being a low pressure system, and the other being a high pressure system. The two systems are driven through two separate concentric shafts, and each shaft can rotate independently of the other.
In the past, heavy supporting structures have been used to support these concentric shafts. In order to eliminate these heavy structures, it is desirable to support the concentric shafts, relative to each other, with differentially mounted bearings, such as roller bearings. In a differential mounting system, the bearing outer race and the bearing inner race are fixed to their respective outer and inner rotating shafts. The shafts can rotate at different speeds and sometimes rotate in opposite directions. While the bearing must be lubricated, the location of the bearing inside the outer rotating concentric shaft makes it impossible to mount a conventional oil nozzle on a stationary member in close proximity to the bearing.
Because an oil nozzle cannot be mounted near the bearing, a different apparatus must be provided to lubricate the bearing. This apparatus must be capable of introducing the lubricant into the interior region of the outer shaft while the shaft is rotating. Introducing lubricant into a rotating shaft is a common problem, and various solutions are disclosed in the prior art. For example, U.S. Pat. No. 3,085,838--Patterson describes a lubrication means employing an annular dam construction attached to a rotating shaft, into which oil or other lubricant is injected with a nozzle. The dam construction causes the lubricant that collects within the dam to be forced into the interior of the shaft by centrifugal force resulting from shaft rotation. Once the lubricant reaches the interior of the shaft, it is well known to those skilled in the art to transport the lubricant by means of centrifugal forces along a flaring inner diameter to the proximity of a device, such as a bearing, requiring lubrication.
The means described in the aforementioned Patterson patent will effectively introduce lubricant into a device mounted between two relatively rotating concentric shafts. While variations of this system are described in additional patents, the prior art does not disclose such a lubrication system wherein the lubricant is introduced through an outer rotating shaft at a position remotely located in respect to an internally mounted bearing, and wherein the lubricant is distributed over 360 degrees of a circular-shaped interface, thereby more effectively lubricating and cooling the bearing.
The currently employed systems, such as that disclosed in U.S. Pat. No. 3,909,085--Wilkinson, et al., generally direct the lubricant to a region located radially inward from the inner race of the bearing structure. The actual transfer of the lubricant into the interior of the bearing structure is accomplished by feeding the oil through radially extending holes into the bearing inner race. All of the extending holes are positioned in the highly stressed region near the bearing, and high stress concentrations develop around each of these holes. In addition, the lubricant is distributed to the bearing at several individual locations, rather than over a broadly curved interface.
In addition, the lubrication systems described in the prior art do not disclose any means for providing a cooling airflow along the internal regions of the concentric shafts. The high rotational speeds of these concentric shafts often make it necessary to provide an axial flow of pressurized air to cool the shafts and the associated internal structures. This cooling air must be transported between the shafts and exhausted somewhere in the region of the aft end of the outer concentric shaft. It is not practical to exhaust this air at the very end of the shaft because driving connections are made at this region, and the ends of the shafts are therefore necessarily closed. Provision generally is necessary in a lubrication system positioned inside the outer concentric shaft for a dual fluid cross-flow, whereby the lubricant flows within the rotating shaft, and the cooling air flows aft to exit the shaft without disrupting the flow of lubricant.
It is, therefore, an object of the present invention to provide an apparatus for lubricating a differential bearing mounted between relatively rotating concentric shafts, whereby the lubricant is introduced into the outer shaft at a location remote from the bearing, transported within the shaft, and distributed over 360 degrees of surface area.
It is also an object of the present invention to provide an apparatus for lubricating a differential bearing mounted between relatively rotating concentric shafts whereby the lubricant is transported to the bearing without the use of holes or similar structural elements that create stress concentrations in close proximity to the bearing.
It is also an object of the present invention to provide an apparatus for lubricating a differential bearing mounted between relatively rotating concentric shafts whereby provision is made for a cross-flow within the shaft of lubricant and pressurized cooling air.